1. Field of the Invention
The present invention relates to a memory card, and more particularly to a method and apparatus for checking a backup voltage necessary for reserving data in the memory card.
2. Related Art
Memory cards have been well known which each have a general purpose memory such as SRAM, EEPROM, ROM, and the like. The memory cards are used as external memories for electronic instruments, personal computers, metering devices, and so forth, or as recording mediums for electronic still cameras. If a RAM is used in the memory card, it is necessary to continuously supply the RAM with a predetermined voltage so as to preserve data written in the RAM. Therefore, the memory card is mounted with a backup battery for supplying the RAM. A lithium battery or another kind of battery which lasts for a relatively long time is typically used as the backup battery.
In this type of a memory card, if the backup battery is consumed, the data written in the RAM will vanish. Therefore, it is necessary check the condition of the backup battery and to replace the backup battery with a new one, before the back up battery in consumed. In checking the voltage of the backup battery of the memory card, it is determined whether the voltage is above or below a predetermined reference voltage in a battery check section of an electronic device to which the memory card is connected. Such an electronic device is hereinafter referred to as an external device. However, the conventional battery check system using only one reference level has the following problems.
If the reference voltage is set at a high level for safety, the backup battery may be changed even through the backup battery still has a sufficient charge amount to preserve the data for a relatively short time. This is apparently uneconomical and wasteful of the battery. On the contrary, if the reference voltage is set at a lower level, the user is required to prepare a spare backup battery for enabling a quick change of the backup battery at any time.
In order to solve the above-described problem, it is desirable to check the voltage V of the backup battery in two steps. Namely, the voltage V should be compared with two different levels of voltages V1 and V2 (V1&gt;V2). When V&gt;V1, the backup battery is determined to be sufficient. When V1.gtoreq.V&gt;V2, the backup battery is determined to be critical. When V.ltoreq.V2, the backup battery is determined to be consumed.
Although at least two connector terminals are necessary for sending check signals of the battery voltage to a battery check section of an associated external device for checking the battery voltage in two steps, only one connector terminal is allocated to battery checking in memory cards having a specific function allocated to each individual connector terminal. Such memory card is, referred to, for example, in JEIDA's "IC Memory Card Guide Line" Ver. 3 JEIDA: Japan Electronic Industry Development Association) as an I/0 Bus-type memory card, which has an interface connector with 20 terminals, as shown in FIG. 2. In this memory card, one terminal Vbat is allocated for battery checking.